In recent years, systems have been developed to measure damage to a vehicle based on images of a probe in contact with specified points on the vehicle. An example of a system using these principles is the Brewco Wolf automotive collision damage measurement system, which is described in U.S. Pat. No. 6,115,927, the disclosure of which is incorporated herein by reference. As described, a camera senses the direction of light from a light emitter on the probe, from three different locations; and a computer triangulates to determine the position of the emitter. A three dimensional coordinate measuring system, like the collision damage measurement system, generally measures locations of points relative to the coordinate system of the measuring device. In a system such as that for damage analysis and repair, it is desirable for the measurements to be in a coordinate system of the item being measured. In some cases, the computer transforms the determined position of the emitter into a standard coordinate system, and that position is compared to standard data for the type of vehicle to determine the extent of deviation of the measured point from the standard data. For collision analysis and repair, for example, this provides data for use in straightening the vehicle or aligning parts of the vehicle, to insure accuracy.
In such image-based analysis systems, the transform to the coordinate system of the measured object generally involves first measuring locations of some reference points on the item being measured. The locations of these references points relative to the coordinate system of the image sensor(s) is used to transform coordinates to a coordinate system of the measured object. If the measuring device moves relative to the measured item it is necessary to re-measure the reference points so that a new transformation formula may be determined.
Adding a reference frame to the measurement system removes the need to re-measure the item reference points when there is a relative movement between the measuring device and the measured item. The reference frame is a device that is mounted in a way that is stationary relative to the item being measured, during the movement of the measuring device. The reference frame is measured continuously by the measuring device. When the reference points on the item being measured are measured, their location is calculated relative to the reference frame's coordinate system. Whenever any subsequent measurement is taken, the location of the reference frame is also measured; and the measured coordinates relative to the camera are transformed into the reference frame coordinate system based on the current reference frame measurement. If desired, the measurement is then transformed into the coordinate system of the object being measured, based on the previously determined fixed relationship between the reference frame coordinate system and the coordinate system of the object being measured. Now the measuring device can be moved relative to the item being measured without affecting measurements in the measured item coordinate system.
In such applications, the reference frame requires at least three points defining a plane. The system described in the U.S. Pat. No. 6,115,927 uses an X-shaped reference frame with clamps for attachment to a vehicle. The reference frame supports four sets of LEDs (light emitting diodes). Each set of LEDs consists of 4 LEDs, with one LED emitting light in each of four orthogonal directions (at 90° angles). Since each LED has a view angle of a little more that ±45°, the X-type reference frame provides visibility of a sufficient number of LEDs for the camera and computer to use it as a reference, from 360° around the subject vehicle. While this does allow for a great deal of freedom to move the camera/sensor system around and measure many points on the vehicle, the reference frame is large and expensive. For example, the relative locations of all of the LEDs must be known to a high precision for this reference frame to function accurately, which mandates machining of the frame to very tight tolerance at a considerable expense.
A low cost device containing at least three LEDs mounted in a plane and disposed in a triangular shape can function as a reference frame. For example, the more recent Tru-Point system from Brewco can utilize a T-shaped frame supporting three LEDs in a plane, as an alternative to the X-shaped reference frame. The data processing system supports both the X-shaped reference frame and the T-shaped reference frame, however, they are not used together. If a wide range of views of the vehicle are desired, the X-shaped reference frame is used. The simpler reference frame has view angle that is limited to a little more the 90° and is used in cases where the wider range is unnecessary. Although this simpler frame is adequate for many applications, sometimes a wider view angle is desired to allow a greater range of movement of the sensor equipment, although the range often need not extend to the full 360° range offered by the X-shaped reference frame.
Hence a need exists for a low cost, simple reference frame system and attendant processing techniques, wherein the reference frame system offers a view angle adequate for a wider range of measurement needs, although it need not always provide as much as 360° total view angle.